Many pathogens have evolved sophisticated strategies to evade autophagy, a crucial cellular defense mechanism that typically targets and degrades invading microorganisms. By subverting or inhibiting autophagy, these pathogens can create a more favorable environment for their replication and survival within the host. For instance, some bacteria secrete factors that block autophagosome formation, while others might escape from autophagosomes before degradation. These evasion tactics are critical for the pathogens' ability to establish and maintain infections. Understanding the mechanisms by which pathogens avoid autophagy is crucial for developing new therapeutic strategies, as enhancing autophagy could bolster the host's immune response and aid in the elimination of pathogenic bacteria. Francisella tularensis can manipulate host cell pathways to prevent its detection and destruction by autophagy, thereby enhancing its virulence. Given the potential for F. tularensis to be used as a bioterrorism agent due to its high infectivity and ability to cause severe disease, research into how this pathogen evades autophagy is of critical importance. By unraveling these mechanisms, new therapeutic approaches could be developed to enhance autophagic responses and strengthen host defense against this and other similarly evasive pathogens.

• Keywords
• Francisella, autophagy, bacterial pathogenesis, host-pathogen interaction, virulence,
• MeSH
• Autophagy * MeSH
• Virulence Factors metabolism   MeSH
• Francisella tularensis * pathogenicity   immunology   physiology   MeSH
• Immune Evasion * MeSH
• Host-Pathogen Interactions * MeSH
• Humans MeSH
• Tularemia microbiology   immunology   MeSH
• Virulence MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Virulence Factors MeSH

The nucleoid-associated protein HU is a common bacterial transcription factor, whose role in pathogenesis and virulence has been described in many bacteria. Our recent studies showed that the HU protein is an indispensable virulence factor in the human pathogenic bacterium Francisella tularensis, a causative agent of tularemia disease, and that this protein can be a key target in tularemia treatment or vaccine development. Here, we show that Francisella HU protein is inhibited by Gp46, a protein of Bacillus subtilis bacteriophage SPO1. We predicted that Gp46 could occupy the F. tularensis HU protein DNA binding site, and subsequently confirmed the ability of Gp46 to abolish the DNA-binding capacity of HU protein. Next, we showed that the growth of Francisella wild-type strain expressing Gp46 in trans corresponded to that of a deletion mutant strain lacking the HU protein. Similarly, the efficiency of intracellular proliferation in mouse macrophages resembled that of the deletion mutant strain, but not that of the wild-type strain. These results, in combination with findings from a recent study on Gp46, enabled us to confirm that Gp46 could be a universal inhibitor of HU proteins among bacterial species.

• Keywords
• Francisella, Gp46, HU protein, histone-like protein, nucleoid-associated protein, transcription factor, virulence,
• Publication type
• Journal Article MeSH

HU protein, a member of the nucleoid-associated group of proteins, is an important transcription factor in bacteria, including in the dangerous human pathogen Francisella tularensis. Generally, HU protein acts as a DNA sequence non-specific binding protein and it is responsible for winding of the DNA chain that leads to the separation of transcription units. Here, we identified potential HU protein binding sites using the ChIP-seq method and two possible binding motifs in F. tularensis subsp. holarctica FSC200 depending upon growth conditions. We also confirmed that FSC200 HU protein is able to introduce negative supercoiling of DNA in the presence of topoisomerase I. Next, we showed interaction of the HU protein with a DNA region upstream of the pigR gene and inside the clpB gene, suggesting possible regulation of PigR and ClpB expression. Moreover, we showed that arginine 58 and partially arginine 61 are important for HU protein's DNA binding capacity, negative supercoiling induction by HU, and the length and winding of FSC200 chromosomal DNA. Finally, in order to verify biological function of the HU protein, we demonstrated that mutations in arginine 58, arginine 61, and serine 74 of the HU protein decrease virulence of FSC200 both in vitro and in vivo and that immunization using these mutant strains is able to protect as many as 100% of mice against wild-type challenge. Taken together, our findings deepen knowledge about the role of the HU protein in tularaemia pathogenesis and suggest that HU protein should be addressed in the context of tularaemia vaccine development.

• Keywords
• ChIP-seq, Francisella, HU protein, HU regulon, PigR, bacterial pathogenesis, histone-like protein, nucleoid-associated protein, transcription factor, virulence,
• MeSH
• Arginine MeSH
• Bacterial Proteins genetics   metabolism   MeSH
• DNA Topoisomerases, Type I metabolism   MeSH
• DNA metabolism   MeSH
• Francisella tularensis * MeSH
• Francisella MeSH
• Humans MeSH
• Mice MeSH
• Serine metabolism   MeSH
• Transcription Factors metabolism   MeSH
• Tularemia * microbiology   MeSH
• Virulence MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, N.I.H., Intramural MeSH
• Names of Substances
• Arginine MeSH
• Bacterial Proteins MeSH
• DNA Topoisomerases, Type I MeSH
• DNA MeSH
• Serine MeSH
• Transcription Factors MeSH

Francisella tularensis is a Gram-negative, facultative intracellular bacterium, causing a severe disease called tularemia. It secretes unusually shaped nanotubular outer membrane vesicles (OMV) loaded with a number of virulence factors and immunoreactive proteins. In the present study, the vesicles were purified from a clinical isolate of subsp. holarctica strain FSC200. We here provide a comprehensive proteomic characterization of OMV using a novel approach in which a comparison of OMV and membrane fraction is performed in order to find proteins selectively enriched in OMV vs. membrane. Only these proteins were further considered to be really involved in the OMV function and/or their exceptional structure. OMV were also isolated from bacteria cultured under various cultivation conditions simulating the diverse environments of F. tularensis life cycle. These included conditions mimicking the milieu inside the mammalian host during inflammation: oxidative stress, low pH, and high temperature (42°C); and in contrast, low temperature (25°C). We observed several-fold increase in vesiculation rate and significant protein cargo changes for high temperature and low pH. Further proteomic characterization of stress-derived OMV gave us an insight how the bacterium responds to the hostile environment of a mammalian host through the release of differentially loaded OMV. Among the proteins preferentially and selectively packed into OMV during stressful cultivations, the previously described virulence factors connected to the unique intracellular trafficking of Francisella were detected. Considerable changes were also observed in a number of proteins involved in the biosynthesis and metabolism of the bacterial envelope components like O-antigen, lipid A, phospholipids, and fatty acids. Data are available via ProteomeXchange with identifier PXD013074.

• Keywords
• FSC200, Francisella tularensis, host–pathogen interaction, outer membrane vesicles, stress response, virulence factor,
• Publication type
• Journal Article MeSH

Primary interaction of an intracellular bacterium with its host cell is initiated by activation of multiple signaling pathways in response to bacterium recognition itself or as cellular responses to stress induced by the bacterium. The leading molecules in these processes are cell surface membrane receptors as well as cytosolic pattern recognition receptors recognizing pathogen-associated molecular patterns or damage-associated molecular patterns induced by the invading bacterium. In this review, we demonstrate possible sequences of events leading to recognition of Francisella tularensis, present findings on known mechanisms for manipulating cell responses to protect Francisella from being killed, and discuss newly published data from the perspective of early stages of host-pathogen interaction.

• Keywords
• Francisella tularensis, innate immune recognition, intracellular replication, phagocytosis, signaling pathways,
• MeSH
• Alarmins genetics   immunology   MeSH
• Bacterial Proteins genetics   immunology   MeSH
• Phagocytosis genetics   MeSH
• Francisella tularensis genetics   immunology   pathogenicity   MeSH
• Host-Pathogen Interactions genetics   immunology   MeSH
• Humans MeSH
• Macrophages immunology   microbiology   MeSH
• Pathogen-Associated Molecular Pattern Molecules immunology   metabolism   MeSH
• Immunity, Innate * MeSH
• Receptors, Cell Surface genetics   immunology   MeSH
• Receptors, Pattern Recognition genetics   immunology   MeSH
• Gene Expression Regulation MeSH
• Signal Transduction MeSH
• Tularemia genetics   immunology   microbiology   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Names of Substances
• Alarmins MeSH
• Bacterial Proteins MeSH
• Pathogen-Associated Molecular Pattern Molecules MeSH
• Receptors, Cell Surface MeSH
• Receptors, Pattern Recognition MeSH